Pawe Adamowicz, who served as the mayor of Gdask since 1998, was taken to hospital on Sunday evening and received emergency care into the night, Reuters reported.
On Monday, Poland's health minister Lukasz Szumowski said that Adamowicz had died.
"We couldn't win," he told The Guardian.
A photograph from the event shows a man holding a knife on stage, just after Adamowicz was stabbed:
Adamowicz, 53, was stabbed by a man who rushed on to the stage during the annual Great Orchestra of Christmas charity event, where volunteers collect millions of euros for medical equipment for hospitals, according to Reuters.
TV footage showed the man shouting that "Adamowicz is dead."
He told the crowd that he blamed Adamowicz's former party, the Civic Platform party, for wrongfully imprisoning him in 2014, The Guardian reported.
Photos from the event show him talking to locals and collecting money for charity.
Adamowicz, a liberal politician, has a wife and two daughters, according to the BBC.
Prime Minister Mateusz Morawiecki said on Twitter: "The assassination of President Paul Adamowicz is a huge tragedy for us." "It is a great evil that awakens condemnation, great sadness and regret," he said.
According to Reuters, Polish President Andrzej Duda will meet with the leaders of the country's political parties on Monday to organize a march against violence and hatred.